An Echinoid. 35 



Draw the reproductive system. 



The Water-vascular System. A tube, the stone-canal, 

 leads from the madreporic plate to a circum-oral canal sur- 

 rounding the oesophagus. From the circum-oral canal a 

 radiating tube passes along each ambulacral tract, giving off 

 branches to the several ampulla. The ampullae in turn 

 supply the suckers. A so-called heart is to be found near 

 the upper end of the stone-canal. 



The Nervous System. A delicate nerve forming a 

 circum-oral commissure surrounds the oesophagus inside the 

 dentary apparatus. Nerves extend from it radially, along 

 the inner surface of each ambulacral series, to the terminal 

 ocular plates, where they end in the eye-spots. 



The Muscular System. A number of minute muscle 

 fibres will be found at the base of each spine, and a more or 

 less complicated series is associated with the dentary appa- 

 ratus. Find the protractors and retractors of the dentary 

 apparatus. The inter-alveolar muscles are arranged between 

 the alveolar plates, which latter hold the teeth. 



Make drawings illustrating the arrangement of the muscles. 



